text:
Is there a practical carbon-free jet fuel?
For example, when I want to use a small turbojet or low bypass turbofan as both the APU and bleed air supplier for blown flaps, ailerons, elevators, (like the ShinMaywa US-2 amphibious plane) and rudders, (and hot wing deicing) there's a lot of internal ducting, and a large number of them are quite narrow, so having soot in them is like having thrombus in capillaries, so it's best that it doesn't contain so much as a single carbon atom in it.
EDIT: OK, the small engine mentioned above is used as a gas generator, not for thrust, that's the main engines' job. For the sake of the argument, let us assume the main engines are not available for bleed air like in real-life large airliners because they are say mounted on gimbals in a tilt-turboprop. If this sounds artificial, how about having a complete set of blown surfaces so the tilt-turboprop can have control authorities in pitch, roll, and yaw at airspeed 0 knots, i.e. when it's taking off vertically. Being a tilt-turboprop means it can not have helicopter-like variable pitch propellers like the V22 Osprey and has to rely on external measures for attitude control.
EDIT: It doesn't matter how weak the alternative fuel is as long as it can support the small jet engine's N1 RPM. It is only for the small engine.

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A different fuel is not necessary. Even the most extreme experiment in diverted jet engine output, the Hunting H.126 did not divert its exhaust flow, it just tapped off compressed air. Its takeoff speed was 52 km/h. All aircraft need forward thrust, and with modern high-bypass turbofans you can get massive amounts of compressed air that's more than enough for blowing any surface you want. The BAe Harrier is a good example; the front nozzles of the Pegasus blow compressed air and match the thrust of the jet exhaust in the rear nozzles.
Fuels roughly fall into 3 categories:

hydrocarbons (everything from methane to jet fuel and diesel). They all contain carbon. Some leave less soot than others: methane is relatively clean. But it's a gas at room temperature, which makes it difficult to store (you need heavy high-pressure tanks).
Hydrogen. Clean, but very difficult to store (needing even heavier high-pressure tanks than methane), and explosive in a wide range of circumstances.
Oddballs like hydrazine and hypergolics. They're all nasty, toxic fuels that require specialized handling which make them unsuitable for civilian use. Hypergolics are two-part fuels that spontaneously combust when in contact with their counterpart.

Some more details on hydrazine:

Hydrazine is highly toxic and dangerously unstable

Anyone involved in fueling a hydrazine-powered vehicle has to wear a full Hazmat suit. That alone makes it difficult to use a hydrazine-powered aircraft on most airfields as it'd disrupt normal operations.
Hydrazine is also inefficient. For jet fuel, you can pull the oxidizer from the surrounding air. Hydrazine has a specific impulse of 220 s, while jet fuel has a specific impulse in the region of 350 s. So you have to carry 350/220 is 1.5 times more fuel to do the same work.
The exhaust products of hydrazine are extremely flammable (hydrogen). You could burn that in a jet engine, which would significantly increase the specific impulse. Hydrogen is a bit finicky, but hydrogen-fueled aircraft have been tested.

I don't see how this answers the question. It is not about hydrazine at all.

That's because you removed the references to hydrazine from the question!

If you read the comments under the question, it was never ever about hydrazine in the first place. It was only used as an example of what might be an answer. Well, it isn't, but saying it isn't isn't an answer.

Note that I first commented here and then removed the sentence from the question. It was not about hydrazine even when the sentence was there.

@JanHudec, I don't see how you can say the question was not about hydrazine, as it started with "Can the likes of Hydrazine be used on a jet engine". It wasn't only about hydrazine, but you can't say either "it was never ever about hydrazine in the first place"

@mins THE LIKES OF is the key phrase here, right there in the bit you quoted. Also, by now the question has been updated by the original asker, without any mention of hydrazine.

@JanHudec: The particularity of hydrazine being that it doesn't need combustion (oxidizer) to generate heat and expand, "the likes of" hydrazine are monopropellants, and removing this bit from the question changes completely the question. While I understand why you did it (and agree), the question was about monopropellant and lack of carbon.

@mins, the question also said it does not have to be fuel that burns with oxygen, but might be a monopropelant, clearly including non-monopropelants.

May I point out that hydrazine does not have to be a monopropellant?

This question is based on a fundamental misunderstanding: by definition, NO bleed air system has combustion byproducts in it. None.
Bleed air is tapped, or "bled", from the compressor section before fuel is introduced in the combustion chamber. Which makes sense when considering that bleed air is generally used to run Air Conditioning packs... you don't really want exhaust fumes making their way into the pressurized cabin!
Any system that IS taking exhaust gasses & doing something with them, is by definition NOT a "bleed air" system.
The solution for the underlying question is similar to what's used in the 737 NG: an APU that burns standard, plentiful, reliable jet fuel, and which turns a generator, and a load compressor. That compressor provides the bleed air to the jet's pneumatic systems, and plenty of it, and quite efficiently.  When you put a bleed air load on that APU, the EGT rises, but not because you're stealing cooling air, but because the APU is now burning more fuel to do more work. Great system, works well.
But at the end of the day, combustion byproducts are always going to be undesirable to have in a pneumatic system, even if it's "only" a constant supply of water vapor. No fuel will do what the OP envisions, but a different approach toward producing the high-pressure air will work just fine.

What about tiny oil particles that escapes the bearings of the fan (in turbofans) and compressor and burns in the compressor before the bleed stage? I believe to have read this is a health concern for airline crew, albeit maybe small, and has been researched. But, even if these combustion byproducts are miniscule, could they accumulate in the ducting over the long service life an airliner?

@Erik The point isn't that bleed air has 0.000000000% contaminants in it, but that it's bled upstream rather than downstream of the combustion that drives the APU, and thus doesn't have (discernible amounts of) any byproducts of that combustion. The original question misunderstood this relationship.

Also, if the concern is that you'd get the "single carbon atom" from lubricants, then switching fuel - as the question asks - won't change that. You could, maybe, alleviate contamination from carbon-based lubricants by switching to something silicon-based, without changing the fuel itself.

I understand. Was not looking to change fuel to solve the carbon based lubrication droplets, but I was curious if the lubrication was a concern at all. But I assume you'd get as much, probably more contamination from simply drawing in contaminated airport air with exhaust etc already in it while on ground, which I picture is where the APU is typically operated

To answer the question as stated in the header, hydrogen powered aircraft have been tried with some success (they fly since about 1989). Many problems need to be solved there but seems less issues than with hydrazine. Burning hydrogen produces just water, so the fuel should be carbon neutral. However not all technologies used to obtain the hydrogen are carbon neutral.
Some burning metals like magnesium "slurry fuel" have been tried in about 1950, with the claims it may produce more thrust. Looks like these works did not advance till the flyable prototype.
Electric aircraft can be carbon neutral, depending on how was the electricity obtained. They also already fly, from as early as from 1917, but even as of 2019 mostly just as technology demonstrators.
Nuclear powered aircraft have been thought about in the past but none were built.

At least according to wikipedia they got quite a bit further than "thinking about" nuclear powered aircraft, running engine tests on the ground and radiation tests in the air, but they never put it all together and had the plane actually fly on nuclear engines.

Hydrogen is usually produced from methane releasing co2 in the process.


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Are WWII turbochargers reliable?
How reliable are WWII turbochargers? Does adding them to an otherwise reliable engine, e.g. the P&W Double Wasp and the Allison, decreases the overall reliability of the engine? Are they well made enough to sustain the heat of the exhaust (my guess from rough calculations under the presumption of adiabatic combustion puts it in the 800K-1200K range) without creeping? My impression is superalloys are not around until about 1950s.
Is the relative unreliability of the turbochargers (turbine wheel to be exact) the reason why the great majority of high-performance WWII engines opted for supercharging as supercharger operates in the cold end of the engine only?

If you add more parts, you add more opportunities for failure. Also, raising the pressure level increases loads on most parts of an engine. So yes, adding a turbocharger, even a modern one, will certainly lower reliability. Supercharging is less efficient, but much less complex than turbocharging an engine.

Note that many WWII turbochargers didn't have automatic waste gates. Pilot error could damage the turbocharger and/or engine.

Early turbojet engines (which also have turbines running in hot exhaust gases) were very unreliable and had short lifetimes (on the order of 10 hours between rebuilds for early Me-262, IIRC), so expecting something similar for turbochargers is not unreasonable.

Supercharging was mature technology at the start of the war while the turbocharger was something quite new.  The P-38 used the then radical new General Electric turbocharger (along with the B-17 and P-47) which was quite reliable in itself but the system couldn't tolerate back pressure, which is why the turbine wheel is exposed on the Lightning's turbos on top of the booms - that's where the exhaust comes out.
In any case, the Lightning's many bugs included boost system related issues, but this was mostly due to an ineffective intercooler system in the leading edges that didn't drop the air charge temperature enough.  The tetra ethyl lead in the fuel would actually separate out and lower the fuel's octane rating on its way to the cylinders, causing detonation and blown engines.  This wasn't the turbo's fault and was mostly fixed by the K and L models with the big chin scoops for the new intercooler.  I don't believe the turbocharger unit itself was a major maintenance headache for the P-38.
That's the thing about the Lightning.  In its L model form it really was the F-22 of its day.  It could outclimb just about anything.  With the hydraulically boosted ailerons, at high speed it could out roll single engine fighters in spite of the mass of those booms.  With counter-rotating props it had no torque effects with power changes and was jet like as a gun platform. Compressibility problems were mitigated with dive flaps.  All the more amazing as a pre-war design.  Kelly Johnson really was a mega-genius.
But the turbocharger and intercooler system, with yards of ducting, along with two of everything in the booms, made it very complex and expensive airplane compared to say, a Mustang with its Merlin's simple two speed supercharger, so you could say it was questionable whether it was worth it (there is endless speculation on what a Merlin powered P-38 would've been like).  So to answer your question, cost and complexity may have kept a lot of designers from using GE's turbo system, more than reliability.

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What would happen if an aircraft were to touch down on EMAS?
What would happen if a jet (say, a A330) were to touch down on the Engineered Materials Arrestor System (EMAS) just before the runway, instead of on it?
According to the FAA the EMAS is designed to "reliably and predictably crush under the weight of an aircraft". Will an aircraft touching down with its main landing gear crush and sink through this material as it would if it hit the EMAS after trying to stop on the runway or will the plane's momentum carry it on to the main part of the runway unharmed?

will the plane's momentum carry it on to the main part of the runway unharmed?

An aircraft landing on the EMAS must not suffer control problems.

According to an EMASMAX document

FAA Advisory Circular (AC) 150/5220-22A
  ...

No adverse aircraft affects in the event of short landing
  ...

The referenced document doesn't state this but does say that EMAS should be set back from the runway end and

must not cause control  problems  for  aircraft  undershoots  which  touch
  down    in    the    EMAS    bed.

According to RUNWAY SAFETY AREA PROJECT - IGOR I. SIKORSKY MEMORIAL AIRPORT,
there is only a three inch reveal at the transition of EMAS bed to runway. So a landing gear transitioning this part this part would likely not be catastrophic. I imagine crushed EMAS material still has some thickness.

Wow, great find, RGB! I spent far too much time looking through that PDF of the EMAS installation.

You'll probably sink in (and have a bad day.)
The longer answer is that it depends on how you land. Force from a touchdown on a runway can vary from nearly zero (in an extremely light landing) all the way up to twice the weight of the aircraft or more (in a hard landing.)
While it's theoretically possible to land an airplane very lightly on the runway, this normally requires a high forward speed and a very low vertical speed. It's rather unlikely that an incident resulting in landing short of the threshold (on the EMAS) would be at such a low vertical speed and high forward speed. The opposite situation (low forward speed, high vertical speed compared to a normal touchdown) is more likely in a situation of touching down short of the runway, otherwise you'd have probably made the runway.
As a result of this, the kind of landing that is likely to result in a touchdown short of the runway will result in forces that are minimally a significant fraction of the aircraft's weight and quite possibly at or significantly above that weight. Since the EMAS is guaranteed to buckle at less than the aircraft's weight in an overrun situation, it would almost certainly do so in the situation of landing on it.
The momentum of the airplane (specifically, the downward component of its momentum) will actually make it more likely for it to sink into the EMAS on touchdown, not less.
Of course, EMAS isn't designed for anything near the touchdown speed of an airliner. According to FAA Advisory Circular 150/5220-22B, a standard EMAS is designed to stop an aircraft that overruns the runway at 70 knots or less. A landing airliner will be flying much faster than that (Peter's answer here says 160 knots for a 747, for example.) This means that the backwards acceleration applied to the airplane by the EMAS will probably exceed the design forces by quite a lot, making for a quite uncomfortable landing for the passengers and quite likely breaking things on the landing gear (if not just breaking the gear completely off.)
The answer from Peter that I linked above also mentions that, in a hard landing where no flare is performed, the average force applied to the runway during touchdown would be just under twice the weight of the aircraft in the case of a 747, so that helps to provide a rough idea of the upper bound of the vertical forces imparted on the surface by an airliner at touchdown relative to the aircraft's normal weight.

If the aircraft sank in and then reached the end of the EMAS (i.e. the start of the runway), it'd probably shear the gear right off, since there would be no ramp to get the gear up out of the EMAS & onto the regular pavement, and still a lot of momentum on the fuselage itself. Bad day.

@RalphJ Good point. And that's assuming the gear hasn't already sheared off from running through the EMAS at more than twice the designed speed.

The FAA. Advisory Circular on EMAS states: "The EMAS must not cause control problems for aircraft undershoots which touch down in the EMAS bed." It shouldn't cause all the problems and damage you describe.

@TomMcW It seems to just say it won't cause you to lose control, not that it won't damage the gear and/or engine nacelles. Interestingly, it uses that requirement to specify a minimum strength for the EMAS material, which admittedly seems a bit counter-intuitive. I am now curious, though, how broadly the FAA defines "cause control problems." I'd think the inability to accelerate forward would be a "control problem," for example, but that seems to be a rather necessary consequence of traveling through EMAS. Maybe they mean only directional control?

@TomMcW Note that the same AC (which I linked in my answer) also says, "To the maximum extent possible, the EMAS must be designed to decelerate the design aircraft expected to use the runway at exit speeds of 70 knots without imposing loads that exceed the aircraft's design limits, causing major structural damage to the aircraft, or imposing excessive forces on its occupants." The speed of a landing airliner is much greater than 70 knots, so it would seem that all bets are off on exceeding design loads on the gear and/or occupants.

They specify that simulator testing is sufficient. Not sure how you could really do they with a simulator. I'm sure broken landing gear would suffice for loss of control. Intuitively it seems like it would be a major problem, but if it would turn a landing short incident into a crash they would have gone back to the drawing board. It's intended for entry at 70 kn. My guess is that it's not all that deep, so if you're going 100+ kn you'd slosh right through it and there's probably a ramp up to the hard surface

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Why have high lift devices on both leading and trailing edge?
Trailing edge devices such as flaps increase lift by raising lift coefficient at s given AoA. Leading edge devices such as slats increase lift by raising stall AoA.
The former is nice because it allows the nose to point lower and give the pilot a better view during landings and takeoffs, and increase tailstrike clearance. The latter does the opposite.
Why have both on aircraft, as is done today? Why not just use bigger flaps? Wouldn't that be safer and simpler?

Bigger flaps and leading-edge devices would produce an even bigger benefit, of course...

Slats increase the stall angle, flaps do not, that is the key issue - safety margin. Plus on aeroplanes with high wing loading even triple slotted fowler flaps may not cut the mustard, slats are simply required additionally.

Pic from this answer, showing that the stall AoA actually decreases in this example. The second pic in the referenced answer shows the curve with both slats extended.
There are aeroplanes with only flaps, the Fokker 28 was one.

Flaps decrease stall AoA.  Importantly, they are generally mounted on the inside of the wing, effectively washing out the wingtips (so they are less prone to stall first).

Slats are a necessity when the wing uses slotted flaps or Fowler flaps for maximum lift. Those flaps cannot reach over the whole span of the wing to leave room near the wingtips for ailerons.
Once the flaps are extended, they increase the incidence of the wing along the flap span. This leaves the outer wing at a lower incidence, but in the vortex field of the inner wing. This vortex field induces a high local angle of attack over the whole wingspan, which would produce a local stall at the wingtip, where the ailerons prevent the use of slotted flaps. This stall would in most cases be asymmetric: One wing tip loses lift while the other barely manages to get along with the high angle of attack. The result is a strong rolling moment which will even increase once the aircraft starts to roll, because the local angle of attack at the down-moving wing will increase further. At this condition, the ailerons are of little help, so the roll cannot be controlled.
Now consider when flaps are used most often: Close to the ground, during take-off and landing. Then imagine what happens when the airplane rolls over one wing uncontrollably. To avoid this, slats are very helpful: They give the wing an extra stall margin by shifting the stall angle up.
Aircraft with lower wing loading normally use less powerful flaps, like simple camber flaps or split flaps. Since they change circulation less, stall on the outer wing can be enough delayed by washout and reduced taper, so leading edge devices can be avoided.

So as @ymb1 implied, if someone figured out how to Fowler flap the whole span slats would not be needed?

Because the angle of attack sensor is mounted on the fuselage, and the pilot is usually there too. Changing both front and back allows the chord AoA to remain the same.
Flaps are valuable on approach because they provide greater forward visibility for the same AoA, literally "drooping" the nose.  The pilot must realize the plane will stall at a lower AoA with flaps deployed, especially at the higher flap settings.  This is why on a go-around one must be careful not to "pull" excessively, even after full power is added.
Flaps also increase drag.  The Slat greatly magnifies the forward, top suction peak, greatly improving the lift to drag ratio.  Together with flaps, they create undercamber as well, also increasing lift, as seen with the DAE-21 airfoil.  This form sports a L/D of over 150.
But Reynolds number is critical.  On smaller models of lower Re values, flaps only may produce better results.

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What makes spiral instability less harmful than Dutch roll?
The FAA Pilot's Handbook of Aeronautical Knowledge says on page 5-20:

Because it is more desirable for the aircraft to have “spiral
  instability” than Dutch roll tendencies, most aircraft are designed
  with that characteristic.

What are the grounds for that?

Because spiral instability is easier to address by the pilot than Dutch Roll tendencies.
Spiral instability is instability about the longitudinal axis. For example, spiral instability means that if the right wing tip moves down, it continues to move down rolling the plane to the right. Its simple to detect and address with left aileron.
Additional information on spiral instability available here: http://www.faatest.com/books/FLT/Chapter17/SpiralInstability.htm
Dutch Roll causes cross-control-type movements across two axes, the vertical and longitudinal axis, such that when the airplane rolls right, it yaws to the left, then it swings back the other way and rolls left, while yawing to the right. This movement is difficult for the pilot to counteract and the inputs from the pilot can actually amplify these movements.
Additional information on Dutch Roll tendancies available here: http://www.faatest.com/books/FLT/Chapter17/FreeDirectionalOscillations.htm

Thank you for your answer. Are there any recommended recovery procedures to break Dutch Roll, like "power idle, full opposite ruder, briskly yoke forward" for a spin recovery?

@lemonincider I've never flown a plane with Dutch Roll tendencies. I usually fly smaller airplanes. I believe that it is more of a problem for higher performance jets with sweptback wings. I believe reducing speed is how you address it. Pilot inputs often makes Dutch Roll worse. http://www.boldmethod.com/learn-to-fly/aerodynamics/dutch-roll/

Thank you very much for your informative replies and the link!

"Spiral instability is instability about the longitudinal axis" ... is it? The spiral mode is a lateral-directional eigenmode, decoupled from the longitudinal modes (short and long period). As you correctly state it causes increasing roll, so I find the longitudinal part misleading at best.

@AEhere, I think he is referring to motion in longitudinal AXIS, rather than longitudinal modes, in that case it’s roll. And that’s true it’s a lateral-directional mode of motion.

@Kolom true, but also about the vertical axis, since roll and yaw are coupled. It is not outright wrong, just misleading due to the choice of words.

@Devil07 "simple to correct with aileron", not if you are near stall.  Perhaps better with aileron and rudder.  This would be more coordinated and generally safer.

@Devil07 concentrated the answer on complexity of the movement (1D vs. 2D).
I'll talk about the response times, which will be different between isntabilities. Typically, the response time of the spiral mode will be longer than that of the dutch roll.
This makes it easier to control spiral for the pilot, which has more time to:

Identify the situation
Prepare the appropriate command mentally
Synchronize its commands to the phase of the instability
Apply the right amount of control force

In the case of faster instabilities, the pilot could have a hard time synchronizing its input to the right phase, and may in if failing to do so actually degrade the situation by adding pilot-induced perturbation.
Finally even when the airplane stays controlable and recovers, faster instabilities are more uncomfortable, as our incomfort "ceiling" seem to decrease with the square of the frequency of the oscillations. Since the spiral doesn't even always produce oscillation, the problem of comfort is never even brought up.

Well, spiral instability does not have any phase as it is simply diverging, not oscillating. So there is no need to synchronize the commands.

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How to log pattern work time during X-country?
I am working on my IFR part 61 and must have 50 hours of X-country as PIC.

Lets say my instructor and I fly to an airport that is 55 miles from my take off airport - it takes us 0.8 hours from take off to land.
Then, on our way back, we do pattern work at another airport - 0.4 hours to fly and 0.7 hours for pattern work.
Then, final leg of flying - 0.3 hours.

Total flight time - 0.8 + 0.4 + 0.7 + 0.3 = 2.2 hours
What is the best and legal way to log this 1.5 hours of X-country, in my log book? If I could, I would like to take advantage of the 0.3 + 0.4 hours during my return flight as X-country time.

Possible duplicate of FAA: May I log as cross-country a flight with two or more short legs? And this question might be relevant too.

You can log it all as XC. This is quite common. XC is all flight time to an airport more than 50 NM away from origin, even if you don't spend it flying in a straight line.


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Engine Oil Consumption limits part of certification?
Does anyone knows if Engine Oil Consumption limits are part of the engine certification requirements?

Hi and welcome. What certification requirements do you mean to ask about?

Indirectly yes, the FAA engine certification guidelines can be found here. The FAA does not really care how much oil you burn so long as you specify it in the documentation and the installation can provide enough to safely run the engine with some margin on top of that.

Paragraph 23.1011(b)
The engine requirements for oil quantities and temperatures are
  established by the engine manufacturer and may be found on the Type
  Certification Data Sheet or in the engine installation manual. The
  airplane installation must not cause the engine limits to be exceeded
  under normal operations.
Paragraph 23.1011(c)
Paragraph 23.1011(c) requires that, “The usable oil tank capacity may
  not be less than the product of the endurance of the airplane under
  critical operating conditions and the maximum oil consumption of the
  engine under the same conditions, plus a suitable margin to ensure
  adequate circulation and cooling.” In other words, there needs to be
  enough oil (usable supply) to allow the engine to consume the usable
  fuel. This should be substantiated by a quantitative analysis. In
  addition, a suitable oil quantity margin for system circulation is
  necessary for all engine installations. The oil system should be
  capable of maintaining the engine within its operating limitations
  (i.e., oil temperature with the minimum oil quantity provided for
  circulation). Cooling tests do not need to be performed with a low
  usable oil supply provided the oil is properly cooled at all oil
  levels.

(To avoid copying all the text here) The document explains how to calculate these numbers as well under Specific Oil Consumption (SOC) and further under Single-engine Installations/Multiengine Installations. The content of Paragraph 23.1011(d) as well as Paragraph 23.1011(e) discuss feeding and pumping the oil around as needed.

Yes. Usually there is an oil consumption maximum published as an operating limitation. Which can be surprisingly high.
The GE CF-34 used on Embraers and RJs allows up to 189 cc/hr or about a fifth of a quart per hour.
The Lycoming O-290 4 cyl piston engine in my plane allows up to a quart in 2.5 hours (!).


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How do navigation displays depict turn anticipation?
I'm working on a navigation display for a simulation and I want to know if someone can explain me how the navigation display displays the "curved lines" for fly-by waypoints.I know how to calculate the distance needed for turn anticipation and the turn radius but how would a real ND draw this ? I attached a screenshot for the ADMAG2X departure for gatwick from a simulator navigation display but why does the turn start after "KKW02" and "KKW02" is no overfly waypoint ? :)

I think it should be accurate because it's from the FSlabs A320 which is a pretty accurate software and all the other displays from other software look the same but maybe I can find real world screenshot :)

You mention that the KKW02 is not an overfly waypoint, but rather a "fly-past" waypoint. Are you sure that this is how it's coded in the database behind what's being depicted? There are places a fly-over waypoint makes sense, and first waypoint off a runway is often coded that way.

But KKW02 IS a fly-over waypoint! https://tinyurl.com/admag2x

I've found mistakes in the FSLabs, too, so they are not perfect either.

The turn radius can be approximated with the speed for the segment (TAS) and the known normal bank limit (e.g. 25 degrees).

Okay maybe that's my first mistake, because I thought "KKW02" is just a fly-by waypoint.I know how to compute turn radius and the distance-turn-anticipation (dta) :)

Sometimes rendering (and flying) turn-anticipation is not possible depending on the geometric/geographic disposition of the two tracks and limitations on the speed or bank angle. Is the overshoot of the turn past the second track's centerline troubling you and prompting your query?

First, the ND doesn't calculate anything. It just renders the navigation data sent to it from the FMS. Yes, it can pan and rotate and scale the nav data. It also displays the compass rose with heading or track from the IRS/AHRS and various data fields around the edge.
The green line on your ND is the lateral path computed by the FMS. The source data for the path is the flight plan which is a list of waypoints and associated leg types. The FMS builds the path by connecting the waypoints as defined by the leg types (ref ARINC 424).
As you noted, it also builds transitions between each of the legs. They can be fly-over or fly-by transitions. The 'normal' transition is a fly-by. Within procedures defined in the nav database, some waypoints will be coded as fly-over. Others will be fly-over due to the specific leg type. Some legs end at fixes, others end at a 'floating' waypoint such as an altitude.
For altitude terminated legs the FMS can predict roughly where it will be, but ultimately the end of the leg is only known with certainty when the altitude is reached at which time you just did a fly-over of the 'fix'.
In your example, KKW02 is a fly-over because it is altitude terminated. Takeoff legs are typically coded in the database as a VA (Heading to Altitude) or CA (Course to Altitude) leg, commonly terminating at 400 feet above the runway altitude.
The FMS builds the transitions based on the actual geometry of the inbound and outbound legs, bank angle limits from the aircraft manufacturer, aircraft speed, altitude, and what it knows of the wind. The resulting transition will be an arc that is tangent to both the inbound and outbound legs. For large course changes like the one shown in your example, it may require two arcs.
Thus the path is made up of a series of straights and arcs. The straights are defined by their start and end positions (latitude/longitude). Arcs have the start and end positions (lat/lon) as well as the radius of the curve (NM) and the center of the arc (lat/lon).
The display gets the aircraft position and track from the FMS/GPS/IRS and heading
from the IRS. Based on the pilot's selected display range the display plots out the path on the display.
RTCA DO-283 Minimum Operational Performance Standards for Required Navigation Performance for Area Navigation covers the generation of the path. RTCA DO-257, Minimum Operational Performance Standards for the Depiction of Navigation Information on Electronic Maps is the standard for NDs. How the manufacturers actually accomplish it is up to them and they don't share that information.

I suggest simultaneously to display the MCDUs to check the data related to every point on the flight plan therefore for KKW02.

Thank you for the long explanation but I'm actually searching for the explanation how the ND draws the Geometry.Will the curved path always be drawn between 2 points or is just the inbound course and outbound curve important for the curve ? I'm trying to understand the laws for the drawing of the geometry so maybe trigonometry and all that stuff :)

The ND just reproduces the plan generated by the FMS (flight management system). Another transcription of the same data is available on the MCDU for every point of the flight point. Analysis of the displayed data on the MCDUs is very helpful.


text:
What is rotor damping and how does it affect helicopter stability?
What do we mean by rotor damping (in terms of helicopters)?
How is it affected by air density?
How does helicopter stability depend on rotor damping?

Do you have a source for that term? It's quite generic, where did you hear about it?

I will try to answer these questions in terms of structural dynamics.
Please correct me if I say dumb things on other parts of my answer.
You ask about damping. Damping must be considered when dealing with structural dynamics, or vibratory phenomena. In a helicopter rotor, multiple phenomena can lead to vibrations. For instance :

Differences in blade properties (mass mostly, but also stiffness)
Rotor imbalance due to machining uncertainties
Bearings
Engine, which may produce torque oscillations of some sort
Gearbox, which may also produce oscillations (look for gear mesh vibrations)
Helicopter operations (if you have quick flight trajectory modifications, this may lead to increased or reduced lift needs, which induce torque oscillations in the drive line)

So long story short, multiple phenomena can excite the rotor.
As any structure, a helicopter has natural vibration patterns called natural modeshapes, associated with natural frequencies.
However, if you excite a component enough on its natural frequency, this may result in a fatigue failure, hence the utmost importance of damping.
As far as I know, the most visible technology used to increase damping on a helicopter rotor are the blade root dampers. Multiple technologies exist, however they seem to damp the same phenomenon : blade edgewise motions or so-called lead-lag motions or hunting motions.
If you want to have a deeper understanding of these issues, have a look at this article for instance. This page may be helpful as well.
The first paper addresses semi-active dampers to control the blade edgewise bending motions in order to reduce the constraints on the hub. Basically, if the blade was mechanically fixed to the hub, large constraints could occur due to vibration loads, leading to for instance fatigue failure. In the configuration presented in the paper and reproduced here,

the liaison between the hub and the blade is a cylindrical liaison, allowing in-plane rotational motion. The (stiffness-)damper system is then used to absorb the kinetic energy of the blade in order to limit the loads on the hub, which allows to reduce its overall weight.
The second paper provides some more general concepts on the technologies involved in helicopter rotors. In particular this here

is a good schematic of a full articulated rotor.
Other rotor technologies exist : semi-rigid or rigid rotors but they use a different strategy for vibration reduction and blade-hub liaison.

Rotor damping refers to aerodynamic and structural forces that counter blade vibratory motion.
Aerodynamic damping forces are proportional to air density.  The most important one is the aerodynamic damping of flap motion.   As a blade flaps upward, the angle of attack is reduced.  This reduces upward aerodynamic force or, in other words, increases a downward aerodynamic force.  This phenomenon always opposes blade "flap velocity."  It pushes the blade down when it's flapping up and vice versa.
An analogous thing happens with lead-lag motion (within the plane of the rotor).  As a blade moves forward drag increases to "push it back."  When a blade lags drag decreases to help "push it forward."  Unfortunately, this variation in drag is very small and does not dampen lead-lag motion nearly as much as the flap phenomenon described above.  It is for this reason that lead-lag damping is problematic, often resulting in mechanical dampers placed near the root of the blade in the lead-lag direction.
Helicopter blades also have structural damping.  Let's say you grab the tip of a blade and try to pitch/twist it (and you are extremely strong!).  When you release the blade it of course wants to go back to its original, undeflected shape.  It may, however, oscillate about that shape like a guitar string.  This oscillation would quickly become too small to see, not so much due to aerodynamics or any add-on damper, but because the blade material itself will absorb much of the energy.  You can search for "structural damping."


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Does shutting down the engine affect the rate of rotation during autorotation?
If a helicopter's tail rotor shears off while flying then the helicopter's fuselage will start rotating in the direction opposite to the main rotor's rotation direction. During autorotation, if I switch off the engines of the helicopter, then will it affect the rate of rotation of helicopter?

The actual procedure for a tail-rotor failure in a helicopter is to maintain as much forward speed as possible. If you have no forward speed (ie hovering), you need to lower the collective as quickly as possible to remove the torque from the main rotor. After this you follow normal auto-rotation procedures and hope you don't build up too high a turn rate before hitting the ground. Tail rotor failure is survivable under certain circumstances.

with the engine disconnected from the main rotor, no torque is being applied to the rotor by the engine so there is no torque reaction trying to rotate the fuselage backwards. In this case, the only torque being applied to the fuselage is the frictional drag in the main rotor bearing, which will be small. If a tail rotor failure occurred and the fuselage began rotating because of this before the engine failure, then the fuselage will remain in rotation after the engine failure and will slowly lose rotational speed as the main rotor bearing friction acts on it.

Of course yaw stability will stop the rotation if sufficient forward speed can be maintained...

I assumed he'd be autorotating straight down...

Not recommended


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What is the meaning of these Lido abbreviations?
Can someone help with the abbreviations and meaning from Lido flight planning:

RCONT 5 MIN
HLDDST
ADDT

For each of these abbreviations, the flight plan has fuel and time:

I placed a red rectangle around the abbreviations; I Hold Distance is ok, however I still o not understand RCONT

Likely planning factors like "hold distance" and "Additional distance". How much fuel for each abbreviation?

RCONT is route reserve / contingency fuel:
SKYbrary Fuel - Flight Planning Definitions

Contingency fuel is carried to account for additional enroute fuel consumption caused by wind, routing changes or ATM: ATM/CNS restrictions. According to ICAO Annex 6, the recommended minimum contingency fuel is the greater of 5% of the trip fuel or 5 minutes holding consumption at 1500' above destination airfield elevation computed based on calculated arrival weight.

In this case the 5 minute holding consumption is greater than the 5% of trip fuel, so it is the one considered in calculation.

Do you know why it says LFPG (Paris Charles de Gaulles) behind the RCONT line? I understand the EHRD (Rotterdam The Hague) behind the ALTN, that is just the primary alternate, but why list Paris behind contingency when flying from Lisbon to Amsterdam?

@Bianfable this would be guessing, but I suspect RCONT might be specified in this case due to weather forecast for ETA in DST and/or ALTN airports being marginal. In this case a secondary alternate would have to be designated, LFPG being the secondary. Enroute alternate would not apply for tap672 as it is not an ETOPS flight not by a long shot ;)

It is also possible that there was a traffic situation in LFPG area, but this is even more speculative...


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Why don't nose landing gears have brakes?
Nose brakes can help the aircraft in braking and it can also help in reducing the travel on the runway length. So why don't we use brakes in the nose landing gear?

Closely related, maybe a dupe?

Possible duplicate of Are there any aircraft with a nose wheel braking system?

Not a dup. One asks whether some aircraft have NLG brakes, this one asks for the reasons NLG brakes are not common, and to this aspect bogl's answer is quite interesting.

The assumption is wrong. Some planes indeed have nose wheel brakes.
The reason why so few planes have nose wheel brakes is that

The braking force of a brake cannot exceed the weight carried by the corresponding wheel multiplied with the friction coefficient. A nose wheel carries much less weight than a main wheel, because the main wheels are much closer to the center of gravity. There is more force on the nose wheel during deceleration, but still much less than the main wheels carry.
The nose wheel brake itself, as well as stronger structure, mechanisms and wheels would add more weight to the nose of the aircraft.
Nose wheel braking reduces the stability of the decelerating plane. This could be controlled in various ways at the cost of increased complexity.

For the aircraft designer it is a trade off: is is worth extra cost, weight and complexity for reducing the braking distance by a small amount?
Note that there are planes where even some of the main gears are not equipped with brakes, for instance the A380, where no more brakes are required to fulfill the requirements.

It’s not only the brake itself that would add weight. Nose gear strut and attachment would suddenly have to carry substantially higher shear loads so would be heavier, too. Stronger tires might come into play. Plus it could add a dynamic tipping risk, reduce load (and braking action) on main gear etc. etc.

@CptReynolds Tried to include that, thanks!

Also, the nose wheel is not even on the ground for the highest speed part of the landing roll, and lightly loaded for the next part.  At least with the (SEL) planes I've flown, it seems like you get more braking effect by keeping the nose high than you would from nosewheel braking.  Lowering the nose at speed also increases the risk of a prop strike...

So that's why all my planes in KSP ground-loop when landing!

A practical answer for a single pilot plane looks to me like a lack of ways to control a 3rd brake. During a landing the pilot's already using these controls:

Left foot on left rudder & brake
Right foot on right rudder & brake
One hand on the stick
One hand on the throttle or flaps

Unless there's going to be some type of squeeze lever on the stick (like a bicycle brake) you've run out of appendages to operate another brake.
And slaving a nose wheel brake to one of the other main brakes could be dangerous if the nose wheel starts to skid and you lose directional control.


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Why are some parts of the wings not protected by the wing anti-ice system in the B737?

This picture is an explanation on the wing anti-ice system of the B737. As you see from it, the system doesn't protect the outboard leading edge slats and the leading edge flaps from icing. Anybody know why? Is it because icing on these surfaces isn't hazardous, or is it due to some limitations involving the bleed air used in the wing anti-ice system?

Good question! I checked the FCOM: it confirms that this is the case, but it does not say why. Here is a picture of the outboard slat actually icing up.

I read somewhere it was also the case for the A380 (I have to find this source).

@ManuH You are right. From the FCOM: "The A380 is less affected by icing than smaller aircraft, due to the size and thickness of its wings. Therefore, slat 4 is the only part of the leading edge that is deiced."

@Manu H Thank you for the information. It's surprising only the slat 4 is deiced in the A380. I have no idea how many slats are used with the A380, but the fact that only the slat 4 is deiced "due to its size and the thickness of its wings" seems to suggest aerodynamics also plays a greater role with the B737's wing anti-ice design.

@Bianfable yes icing likes sharp pointy projections and if a LE radius is large enough, you don't need to anti-ice it.  As for the outboard slats, my guess is it's because there's no aileron behind it but I'm not sure exactly why.  You can also avoid deicing the horizontal tail just by making the surface large enough to function with an ice load.

@John K That seems like a fair guess, but I'm not sure what makes you believe the slat without an aileron behind it doesn't need protection from icing.

The down aileron changes the camber and effective incidence so the down aileron can trigger a local stall when AOA is at the margin.  This is why students are trained to avoid using aileron to keep wings level during stalls - at the stall the plane will roll into the down aileron - although most modern airplanes are so benign you can do it without ill effect.  I expect that during testing, they found that effects of ice on that section of wing weren't causing controllability issues, so they were able to save a few pounds by eliminating heat to that LE.

@JohnK "icing likes sharp pointy projections and if a LE radius is large enough, you don't need to anti-ice it" Why is that? A stagnation point (line) is still a stagnation line no matter the LE curvature.

@StuartBuckingham there is a term "collection efficiency" that describes the ability of shapes to cause supercooled water to strike and freeze on a surface.  The larger the airflow disturbance due to bluntness, the larger the flow deviation around the object, the collection efficiency goes DOWN.  It seems pretty counter intuitive, but any course you take that covers aircraft icing teaches that.   It's usually an ATPL exam question.  I think it's to do with the fact that the larger "bow wave" you might say, of a blunt surface, tends to carry the droplets around the surface more.

There are always two answers to any question about why something is or is not on any particular aircraft: one regards regulation and the other practical application.
FAR § 25.1419, Ice protection, describes the certification requirement to allow transport category flight into known icing. No specific parts of the aircraft are listed, only that

(a) An analysis must be performed to establish that the ice protection for the various components of the airplane is adequate, taking into account the various airplane operational configurations; and

(b) To verify the ice protection analysis, to check for icing anomalies, and to demonstrate that the ice protection system and its components are effective, the airplane or its components must be flight tested in the various operational configurations, in measured natural atmospheric icing conditions and, as found necessary, by one or more of the following means...

The means listed and their explanation in various Advisory Circulars are rigorous, so the absence of icing protection for the leading edges of the 737 wing tips and roots is certified as adequate. This may seem obvious but regulations have been known to specify a particular solution to a problem, and could possibly have allowed certification without expensive testing if, for example, at least the middle 50% of the wing had a particular type of anti-ice installed.
Since the regs aren't responsible for this particular solution, we look to practical application. Bleed air anti ice is heavy, expensive, and reduces engine power output. Aircraft don't carry anything they don't need, so the question becomes: if you need a whole wing for an aircraft to fly, why do you only need to protect half a wing from ice accretion, and why pick the center section?
The answer to the first part of this new question has to do with critical phase of flight, which in this case is takeoff. The aircraft is heavy, slow and may need every bit of wing and available engine power. Diverting bypass air to heat the wing at this point is doubly punitive. The reason it is not necessary is because the wing (and control surfaces) are assumed to have been deiced/anti-iced by equipment on the ground. Beginning 1981 bleed air could be used to heat the slats during ground operations in the 737, and was automatically deactivated if engines approached takeoff power.
§ 121.629 Operation in icing conditions

(b) No person may take off an aircraft when frost, ice, or snow is adhering to the wings, control surfaces, propellers, engine inlets, or other critical surfaces of the aircraft or when the takeoff would not be in compliance with paragraph (c) of this section.

To satisfy this reg without ground de-ice you would have to heat two thirds of the airplane surface area, including the entire wing, which is wholly impractical. Assumption of ground deice/anti-ice allows design of onboard anti-ice to assume excess lift is available and you no longer need the whole wing. In the case of the 737, you only need to protect half the wing.
So why the center 50%? It's not always just the center, and engine power is (believed to be) the reason.

The NG series outboard slat has no wing anti-ice facility (see photo) believed to be due to excessive bleed requirements. However in June 2005 it was announced that the 737-MMA will have raked wingtips with anti-ice along the full span. This is because the MMA will be spending long periods of time on patrol at low level where it will be exposed to icing conditions.

The 737-MMA (Mult-imission Maritime Aircraft) is a modified commercial 737 airframe, known as the P8 Poseidon and described as

“A bit of JSTARS (Joint Surveillance Acquisition Radar System), a little bit of AWACS and a little bit of MC2A (Multirole Command and Control), but with the added ability to go and kill a submarine.”

Another unique feature of the P-8A is the raked wingtips. These, and the horizontal and vertical stabilisers, are electrically de-iced by electro-mechanical expulsion de-icing systems (EMEDS). EMEDS shakes the ice off the surfaces by using actuators in the cavity behind the leading edge. They can dislodge ice thicker than 0.15cm (0.06in).


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Can someone explain this glideslope question?

The glideslope deviation bar is covering the second upper dot. The required action to meet glideslope beam is to:

What does this question mean?

Welcome to aviation.SE! I changed your question to make the words clearer. But if I made a mistake, please edit it again yourself. The [tour] might help you to understand how this site works.

If you are answering an FAA Knowledge Test question, the correct answer is to level off and intercept the glide slope.
While on the glide slope, pitch controls airspeed and power controls altitude, so you will need to add a bit of power. For small very small deviations caused by turbulence, just let the airspeed vary and use pitch for a short while. For larger deviations you won’t be able to capture the glide slope without adding power.
The amount of correction depends on how far away you are from the glide slope transmitter. Figure 139 from the Instrument Knowledge Test Supplement give you an idea of the amount of correction you need depending on how far away from the runway you are. So if you are near the outer marker, 2 dots would mean that you definitely need to add power, level off and intercept the glide slope. Near the runway 2 dots is less than 100' so a slight adjustment with pitch would be fine, presuming you were holding the glide slope until then.

You’re low.  Add power, tweak the nose up a bit.  Watch rated of descent decrease a couple hundred FPM. The glideslope bar will move down slowly. As it approaches center, reverse the correction by decreasing power and dropping the nose a little. Rate of descent will settle back down to your target. Add about half the power you just reduced to hold it there.  By this time it’ll be time for a new correction.

It's referring to the dots for the glideslope on the Horizontal Situation Indicator. (Shown on the left side of the diagram below)

source
The dots above center indicate the plane is below the glide slope so rate of descent needs to be reduced.


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How is the load & trimsheet form sent to ACARS?
After the last passengers and baggage are loaded onto the aircraft, the pilot in command is given a form called load & trimsheet which shows the passenger/baggage distributions, total weights and cg positions. The form can be prepared manually or by a computer program by the authorized ground staff. This form has the wet signature of the ground crew coordinating the flight and the pilot in command on it and these signatures ensure that the form is legal.
My question is about the process of sending this legal form to the cockpit of the aircraft via ACARS. When the form is transmitted to the aircraft via ACARS, is it still considered to be a legal document without wet signature on the form?
How is this process run by airlines?

When the loadsheet arrives to the aircraft via ACARS it has to be acknowledged and printed. This is accomplished by the flight crew by selecting the proper screen and pressing the respective button or LSK on the MFD (Multi-Function Display for latest Boeing models) or in the MCDU (Multi-Function Control Display Unit for Airbus).
Along with the acknowledgement sent back to the system, goes the timestamp. Legally, this action is considered the wet signature. Then the sent ACARS form is printed in the aircraft printer, so the crew has a backup copy of the loadsheet containing in the header the "ACK" confirmation and the same timestamp sent to the system.
Every airline has different policies regarding the format of the loadsheet and even the procedure to accept and correct the values. that sometimes change in the last minute again, but this can give you a generic idea of the procedure.

Hello Fco, welcome to aviation.stackexchange.com!

Welcome. Do you know a guide that details the whole process such as how to act when system is not running, how many copies of the form to print, how to do lmc on the form and by whom etc.

Thanks! great topics around here and wise readers.

Hi GolfCharlie, all that is always regulated by the Airline Operations Manual. When the system is not running, the flight dispatcher has to bring physically a printed copy of the loadsheet to the cockpit and get it signed by the captain. Regarding the LMC, there is always a limit that the captain can correct manually without requiring the calculation of a new loadsheet (around 2000 lbs. for medium aircraft and 4000 lbs. for wide body aircraft). This is considering less weight in the LMC, if the weight is more, it has to be calculated and sent again by dispatch.

I know i’m writing too late but thank you for the answer. I wish you healthy days.


text:
Does a no-touch policy really work?
In Boeing 787 and Airbus 350 aircrafts, more than half of the aircraft body is made of composite material. The composite structure of the fuselage does not indicate the damage when the body is contacted and damaged by the ground support equipment. That's why an airline company I’m familiar with issued a statement for the entire fleet to the ground staff indicating "do not touch the aircraft while servicing" (a no-touch policy).
How can the passenger mobile stairs be aligned safely from the ground without contacting such high aircraft passenger doors?  Wouldn't it be possible for passengers to fall down from the gap between the aircraft and the mobile stairs, especially in windy weather?
What else can be done not to damage composite structured aircraft fuselage?

Where is this airline?  Asia somewhere?  My spidey sense tells me this is China or similar.

@Sean, « The composite structure of the fuselage does not indicate the damage when the body is contacted and damaged by the ground support equipment », please could you justify this assumption ?

Kindly see those videos for justification on youtube: https://youtu.be/TJofm5MuLoc                                                  https://youtu.be/Ce6fE_eT_vE

@GolfCharlie, very interesting, thank you

@GolfCharlie, so if I understand properly, the parts that do touch the aircraft even though made of material that do not damage the surface of the airframe, might internally cause deep damages.

@user40476 there is a risk that gse, even though unintentionally harshly contacting the aircraft, may damage the internal structure of the aircraft even if it is not visible from the outside. In the event of such contact, it is essential that the area in which the internal structure is checked using special equipment by authorized persons.

@user40476: Don't ask me - I'm just the one who fixed GolfCharlie's punctuation.

Jet bridges are operated just next to the point at which passengers will enter the aircraft. They have proximity sensors that tell the operator if they getting too close. And the bridge doesn't have to physically touch the aircraft as long as the gap on the walkway is small enough to not pose a danger to the passengers, it just needs to get close enough so that the end of the bridge can extend like an awning and cover the entrance. The parts that do touch the aircraft are usually made of material that will not damage the surface of the airframe.


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Is it dangerous to do a skidding turn to lose altitude without increasing airspeed?
I was taking some lessons in a light sport aircraft in South Africa a few years back, my instructor taught me something which I had never seen or heard of before or since. We went through an emergency scenario in which we had to make an emergency landing in a field (not really landing but just going through the motions, of course). I found a place to land and started my descent, but had to loop back around after I passed it. The instructor then told me to give full right rudder with left aileron to quickly descend without gaining airspeed.
Wouldn't this qualify as a recipe for creating a spin and possibly killing myself? Should I actually implement that skidding turn in an emergency to land if need be?

Sounds like a forward slip, which is a pretty standard procedure.

Yep. That was exactly it. Thanks Ron

There is an important difference between a slip and a skid.

This is an extreme example of what your instructor was trying to teach you: https://www.youtube.com/watch?v=wW8GRJI6Kz4. Bigger airplanes can simply deploy flaps to increase drag and reduce both airspeed and altitude without losing control. Smaller planes like the Piper Cub don't have flaps but you can use the entire fuselage to increase drag

@KJP: the link you mention is about turning.  This is about going forward, hence forward slip. It is not the same thing.

Do you still want an answer about the dangers of doing a skidded turn, or has your being corrected on proper terminology satisfied your curiosity?  If the latter, please change the question Title to match what it was that your instructor was doing.  It wasn't a skid.

Since the title speaks of turning, could you clarify as to which direction (if any) the flight path was curving (turning), and which (if any) was the high wingtip?  You've stated that you were holding " right rudder with left aileron", which makes me guess that that the right wingtip was higher, but I can't tell for sure, nor can I tell which direction (if any) the flight path was curving (turning).  As for the latter, I'd guess toward the left-- but I can't tell for sure.

As Ron Beyer pointed out in a comment, your instructor was showing you a forward slip - a pretty standard maneuver for light aircraft.
A spin would involve full rudder and a stalled wing, which usually means pulling the yoke or stick back a lot.  That's why overshooting the base-to-final turn can be dangerous - you likely already have a lot of rudder in and it's tempting to pull back on the yoke to try to tighten up the turn.  An unexpected spin at low altitude is not recommended as recovery tends to be both abrupt and fatal.
As long as you're keeping the yoke more or less centered forward and back and your airspeed up, there should be nothing to worry about.

A slip is a handy maneuver, should be practiced with an instructor at first, and at a high altitude enough for recover in case there's a problem.

Well, there is a thing to watch out for: airspeed indicator error. Fortunately it will read low, not high.

I just want to go back to the OP's concern that rudder one way and aileron the other way is related to a spin - it's my understanding that this is a cross-controlled situation, and that is something that can't be ignored, so (again, I'm saying this from an armchair not-even-a-pilot standpoint) it will influence what you have to think about doing at the same time - i.e. not getting slow or finding another way to approach stall regime.
See also the link KJP gave in a comment for the difference between a slip and a skid: "What is a skidding turn (vs slipping turn)?" - this link includes information about the opposite spin-resistance vs. not-spin-resistance of those two maneuvers.
While slipping on final straight-in may not be an easy or likely way to enter a spin, it's not a completely risk-free proposition (like many aspects of flight aren't) for various reasons and while I wouldn't personally avoid it, I'd learn more about it, talk to my instructor about my concerns, practice it with her up high, and of course read answers to similar questions to get a sense for the widespread Internet commentator feelings on the matter."
Googling "can forward slip lead to spin" yields (among others) results (which I lack reputation to post more of):

http://www.askacfi.com/5189/forward-slip-and-classic-countercontrol-stallspin.htm

Answers and components of answers seem to run a range of things like:

"slips like this are perfectly OK"
"slips like this are probably perfectly OK, depending on aircraft and weather details"
"don't cause a stall and you won't spin"
"even stalling in a slip condition won't/shouldn't cause a spin because you're not yawing"
"slips like this are OK but from this condition you can enter certain other conditions which aren't OK"
"try it way up high and see"

And the FAA "airplane flying handbook" has this (and more) to say: (https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/airplane_handbook/ chapter 8, section "Intentional Slips", pages 11 and 12, which I identified from one of the comments in the above google results):

"Unlike skids, however, if an airplane in a slip is made to stall, it displays very little of the yawing tendency that causes a skidding stall to develop into a spin. The airplane in a slip may do little more than tend to roll into a wings level attitude. In fact, in some airplanes stall characteristics may even be improved."

This does provide an answer to the question, it just doesn't add anything substantive to the previous answer.

Slipping an aircraft is mostly perfectly safe. I say mostly because there are exceptions:

Some aircraft operator's manuals explicitly forbid slipping. This might be because of the general handling or aerodynamic characteristics of the aircraft.
There are other reasons outside of handling why slipping is dangerous to some types of aircraft.
For example aircraft with jet engines, especially fuselage-mounted ones are prone to engine stall when airflow to the engine is disturbed. Other reason could be unexpected results on ie. fuel feeding with sideways acceleration.

Also slipping disturbs the airflow to the pitot-static system, so speed, altitude and vertical speed indications are unreliable while slipping.
That said, as a maneuver (on an aircraft that slipping is allowed with) it is safe and normal use of flight controls.

"Is it dangerous to do a skidding turn to lose altitude without increasing airspeed"
This is a great question as it underscores the importance of approaching at around 1.3 times stall speed.  Also, of critical importance, is your instructor did NOT request a skidding turn, what was requested was a forward slip, although I admit this may have taken me by surprise too.
Anything like this should be learned from an expert and practiced at altitude in a plane that allows for it.  In a Cessna 172, approaching at 65 knots, with 20 degrees of flaps, they are a joy to do.  50 knots would be much more dangerous.
A skidding TURN is dangerous as it creates a much greater speed differential between the wings, prompting a poor soul to "hold the inside wing up" with aileron. This is the killer.
However, in a forward slip, The airplane maintains its heading (straight), creates drag with the rudder push, and controls lateral movement along the heading line with aileron.  In a straight wing GA aircraft, after the rudder push, there is no difference in the wing tip speeds,
just make sure you keep an eye on the airspeed.
The change in glide slope can be fairly dramatic, making this a very useful tool.  Just roll out and straighten out before pulling elevator to "round out" your short final.  Your instructor did not steer you wrong.


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Is there a difference in aerodynamic efficiency between single engine and twin engine airplanes?
I've learned a lot of great information about aviation on this website. One insight I learned from a poster here is that, all else equal, for every doubling of engine power that you give an airplane, speeds increase by about 25%.
However you're not always comparing "all-else-equal" airplanes, for example sometimes I want to compare between a single engine airplane a twin engined airplane. Yes the second engine adds weight and drag, but the factor that I have the most trouble accounting for are the difference in aerodynamic requirements between the airplanes.
My understanding is that regulations require all single engine airplanes to have a stall speed of no greater than 61 knots, while a twin-engine plane could have an arbitrarily higher stall speed. The higher stall speed is directly correlated with higher aerodynamic efficiency, meaning that it should be able to achieve higher speeds than a single engine airplane of the same mass, drag, and power, would be able to achieve.
So I am looking for a simple rubric with which to be able to compare single and multi engine airplanes. I think that there are really two parts here: (1) identify realistic stall speeds for the multi-engine airplane and (2) formula for comparing top speeds between planes with X mass and drag and Y power and Z stall speeds.
Any ideas?

There's a pretty big difference between the engine in front of the hull or in front of the wing. I'm not sure a apples to apples is possible.

@ratchetfreak thanks, it would definitely be illuminating to have a framework for comparing an airplane with a stall speed of 63 knots and one with a stall speed of 80 knots. If you are building an experimental or kit plane you don't have to obey the standards so you could build a single engine plane with whatever stall speed you want.

The stall speed limit is 61 knots, as demanded in FAR part 23.49.

Your question really needs further bounding to be able to accurately answer.  I will discuss a few aspects.
First @ratchet freak is right on the money.  What engine mount are we talking about? A podded engine on the single and twin (or multi, since your one of your questions mentions multi) or some different configuration.
In the late 70's and early 80's there were a bunch of academic and not so academic papers and articles written on the topic.  Generally the focus was on the Cessna 336/337, O-2 and Defiant type aircraft.  There were also papers on a loss of engine scenario.  This got rekindled for the Voyager aircraft design and promotion.  The Voyager had a L/D of 27. And it was intended that one engine shut down in cruise flight.
The 61 knot rule for single engine is said to be influenced by crash survivability. I get that, but I will editorialize that several studies have shown that the chance of something bad happening with the loss of an engine is greater in a multi.
Top speeds, stall speeds and mass (weight) are different parameters, subject to different trades.  Furthermore, putting this into the context of existing, production or some arbitrary subset of the universe makes your question hard to answer.
The real answer is that one can design for what you need.  You can have a very slow stall speed in a single or a multi, what would you like?  You can have a nice L/D with any number of engines, what would you like?  And you can haul lots of stuff, although there are not heavy singles around. But to generalize all three parameters, of these non-linear trades, into one rubric, would likely challenge even the most mathamatically talented aerodynamcists that I know.  If you can tighten the bounds on the problem, you might get a better answer.

Thanks. My motivation is to answer the question, "if I had 2 planes of the same cabin volume and weight, same wing shape, and with the same total power, but one had the L/D for a 61 knot stall speed and Y max speed, and one had X higher stall speed, how would I compute the max speed of the second plane?" Really I am trying to understand the tradeoff between going single or double engine regarding the regulatory limits on stall speeds that pertain to single engine planes, and to what degree the tradeoff of being able to have a lower drag wing compensates for the performance tradeoffs of twins.

And yes I understand that choosing a lower drag wing has other tradeoffs. I have read several articles that reference those crash statistics showing twins having higher accident rates than singles. Imagine a scenario where those crash statistics are mostly attributable to the skill requirements of piloting a manually operated, twin-engined plane, with an engine out, and that with modern fly-by-wire control systems that make piloting a twin with an engine out arbitrarily simple, that there is a new equation for evaluating the value of twins.

So not only at least as safe as singles, but with new aero diesels lowering the percentage of fuel cost to total operating cost.

Are you asking about the impact of a higher wing load on cruise speed?

As for the stall speed of the multi, how about 79 knots (~30% higher than the single). Piper Aerostar had an 81 knot stall speed so 79 is feasible even for a light twin. As for the engine placement, whatever is easier to compare. Maybe assume that both the single and the twin have their engines concealed in the rear fuselage so that the aerodynamics are more similar. My intuition is that assuming the engines are in the rear fuselage and pushing propellers that are closer to the centerline (than they would be if wing mounted) makes engine out controllability less of an issue as well.

Maybe. I don't know enough about whether wing loading is the only determinant of performance. I am trying to explain what I want to learn. How to compare, for example, a single vs twin where you know that the single will be limited to a wing that provides enough lift to achieve a 61 kt stall speed while the twin could have a wing with less drag.


text:
Can I become a pilot if I have a medical/dental problem?
Can I become a pilot if I have a dental problem (I don't have 6teeth I have artificial one)  and I broke my chin bone I have a steel plate there and I really want to become a pilot is this okay? can I still become pilot as I don't have any other defect in my body ?

What country are you in?  The regulations may vary depending on your location.

Under FAA regulations (which most people in the world can train under if they come to the USA) you should be fine. There are no dental requirements to be a pilot or obtain a medical certificate (as far as I know). You can reach out an AME in your local area if you are concerned, they should be able to answer the question for you. The only orally related regulations are that you be able to speak english proficiently and clearly. As long as you can do so you should be fine.

This is a link for the FAA i.e the US one:
https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/ame/guide/standards/
, Europe has other standards, I am not a AME ( Aeronautical Medical examiner) but I would think it poses no problems to have dental issues for an initial, in Europe we needed to have a certain mastication coefficient that is you can chew food properly that’s all as far as i remember.


text:
What does "DO" in "DO-160" stand for?
What does "DO" in "DO-160" stand for? Is it short for document? I have searched in internet but it only mentions the document details.

Is it short for document?

Yes. From Wikipedia:

The designation DO stands for "DOcument".

And the EUROCAE (European Organization for Civil Aviation Electronics) equivalent of RTCA DO-160 is ED-14.  ED stands for EUROCAE Document.  Sometimes engineers actually do things that make sense.


text:
Why are Li-ion batteries not being used in aircraft but are widely used in electric vehicle industry?
This might seem a very naïve question. Maybe the comparison of electric vehicle to aircraft is hugely inappropriate. But I couldn't find a clear answer in web search.
Lithium-ion batteries have a high power-to-weight ratio, high energy efficiency and good high-temperature performance. Even then they are not preferred to be used in aircraft.
If safety is the concern, aren't there enough protection mechanisms which have enabled their widespread proliferation in electric vehicle industry.
Why is the weight to power-to-weight advantage not being exploited by aerospace industry? What are the other aspects to be considered- extreme environmental conditions or so?
Edit: I am asking about the battery usage as in typical aircraft, not for electric propulsion. Sorry for not being clear earlier

If you're asking about using batteries as a replacement for conventional jet fuel, this question may be helpful (or even a dupe).

If you are asking about batteries for propulsion this question could also be a dupe: https://aviation.stackexchange.com/questions/45040/what-energy-density-is-required-for-the-batteries-in-order-to-make-an-all-electr/45044#45044

Li-Ion has a high Power/Weight compared to other battery chemistries.  But when compared to jet fuel, Li-Ion batteries have about 50x less energy per weight.  Jet Fuel is still among the most energy dense material we have.

@abelenky: * (non-nuclear)

Note that this question was previously asked on EE.SE: https://electronics.stackexchange.com/questions/604059/why-are-li-ion-batteries-not-being-used-in-aircraft-but-are-widely-used-in-elect

high power-to-weight ratio, high energy efficiency and good high-temperature performance - perhaps when comparing to lead-batteries, but compared to fuel, all of these are 'laughable' instead of ' high'... also vehicles don't care fore safety in a way that's even comparable to aircraft

Eviations new electric aircraft 'Alice' which uses a 980 kWh lithium-ion battery made of 9,400 battery cells with a total weight of 3.8 tonnes, makes up 60% of the aircrafts weight.

Boeing put them in 787. They caused the type to be grounded for a bit over two months in 2013 due to battery fires.
Li-ion battery fires of consumer electronic devices are also semi-regular occurrence on passenger flights, and the reason they are not allowed in checked luggage so the cabin crew can deal with the fire when it happens.
Boeing did manage to improve the battery and get it certified for the very strict requirement of failure rate of less than 1 per 10⁹ hours, and they are used on that plane. But it's much stricter than what is required on ground vehicles, because land vehicles can be easily abandoned when they start to burn, which is not possible in airplane. In many cases the work to ensure the battery is safe enough for a plane is simply not worth it.

Airline regulations are also the reason that Laptops have a maximum battery size. Back in the early 2000s there were laptops with very large batteries. Then airlines limited how much lithium you can carry on board which forced manufacturers to limit laptop battery sizes.

And Li-Ion fires are very hard to extinguish. Those shooting jets of flames can occur in a vacuum because the packs themselves provide enough fuel and oxygen to feed the fire. The stuff also reacts with water and air.

@Nelson. Though at least for the batteries in consumer electronics, throwing them in a bucket of water is good enough, and is the standard procedure for cabin crews. I believe the lithium itself almost never gets exposed to air or water and it's mainly the package and electrolyte burning that can be extinguished and a bucket of water is enough to absorb the heat a short circuit in a phone or notebook battery can release. But big batteries are indeed a problem.

Obligatory XKCD.

@slebetman laptop batteries themselves generally fall under the 100Wh limit, even my performance Alienware had a 96Wh battery. though with airline approval you can bring 2x 101Wh-160Wh batteries on board.

Li-ion batteries are actually allowed in checked luggage, but with certain restrictions (only in equipment, no spares, devices entirely off). See https://www.iata.org/contentassets/6fea26dd84d24b26a7a1fd5788561d6e/passenger-lithium-battery.pdf

@jcaron Right, it is loose Li-ion batteries that aren't allowed in checked luggage. Too much stuff has built-in, non-removable Li-ion batteries to feasible ban them entirely from checked luggage and the risk is generally pretty small from installed batteries in a device that is turned off.

@SebastianLenartowicz just another point showing just how ludicrous our current security theater is. :/

If we're talking about using the batteries for propulsion (and we're talking about aircraft that can carry at least one person) then it boils down to, in a word, weight, or in two words energy density. Li-ion batteries have a pretty good energy density for a battery, but they aren't even in the same league as aviation fuel. Heck, they aren't even playing the same sport.
Something like Avgas has a Specific Energy of ~44.65 MJ/kg, Jet fuel ~43.15 MJ/kg and at the moment Li-ion batteries as used in electric road vehicles are around 0.72 MJ/kg so for the same take-off weight you're going to get a frankly pitiful amount of range.
That's not to say it isn't being looked in to - e.g. Vertical Aerospace's proposed
VA-1X
This paper does some modelling around the concept of an e-VTOL aircraft like the VA-1X, and for a Gross Take Off Mass of 2500kg (about the same as a fully laden Cessna 172) you get less than a hundred miles of range. And that's when the batteries are new - once you start putting charge cycles on them the figure is only going to drop (no pun intended).
The weight of the batteries needed to replicate the range/performance of even a modest passenger jet is mind-bogglingly huge.
Of course that doesn't mean we'll never see battery-powered aircraft - Tesla have been teasing a substantial step in pack energy density for a couple of years now, and an alternative battery chemistry of Lithium-Sulfur  (Li-S) offers a potential solution as they're already hitting nearly double the energy density of Li-ion (and improving rapidly) and if they can solve or mitigate the current issues Li-S has with rapid degradation and power-to-volume ratio then they could be very promising.
Texas aircraft are working on a Li-S-powered electric version of the Colt that could have a range of around 200 nautical miles

Even if the Li-S battery can double the energy density of a Li-ion battery, it's still a paltry 1.42 MJ/kg which is still only 1/30th the energy density of Jet fuel. That's a long way from commercial use on large passenger or cargo flights. The Colt would end up being much like the Tesla Roadster - an expensive toy for the hyper rich to brag about, more than a practical vehicle for transport.

@FreeMan I agree it's got a long (long) way to go before it becomes viable for real-world practical use. Li-S is supposed to have a theoretical energy density of 2,700Wh/kg which is around 10x what we're currently achieving with Li-ion. Still not up to jet fuel, would at least get something vaguely practical.

It's not a completely fair comparison to say that it's 1/30th @FreeMan, electric motors are lighter than jet engines so the weight savings improves that a bit, but it's still a huge disparity. Plus, I don't want to be waiting for 8 hours while they recharge my plane.

Don’t forget that electric motors have a higher efficiency, need less cooling, don’t need a fuel pump and so on. I guess you also don’t need constant speed propellers with electric motors. So it’s not as bleak as the 60:1 difference in energy density makes it look. Otherwise the electric planes which do exist would perform much worse.

The "but the engines weigh less" argument is rather irrelevant. Takeoff weight for a modern jet is about 30% fuel and 3% engine. If your alternative energy source is only 10% less efficient, no amount of engine weight saving can compensate. Batteries are not 10% but 98% less efficient.

Don't forget, once you burn avgas, it becomes reaction mass. You have to drag the reusable, discharged battery with you to the destination as deadweight.

@obscurans Exactly. The real difference is more like a factor of 100x, since you don't have to carry the weight of fuel you've already burned, while you do still have to carry the entire weight of discharged batteries for the whole flight. You only carry an average of a bit over 50% of the fuel mass for the duration of the flight

This is ignoring the fact that electric motors are typically more than 90% efficient compared to around 55% for engines.

Actually, Li-ion batteries are used in aeroplanes - the grounding of the Boeing 787 in 2013 was caused by problems with them. From Wikipedia:

In 2013, the second year of service for the Boeing 787 Dreamliner, a widebody jet airliner, several aircraft suffered from electrical system problems stemming from its lithium-ion batteries. Incidents included an electrical fire aboard an All Nippon Airways 787 and a similar fire found by maintenance workers on a parked Japan Airlines 787 at Boston's Logan International Airport.

They are also used in the Airbus A350, as FlightGlobal mentions in this archived article:

Despite having similar functions, the Boeing design contrasts significantly with the lithium ion batteries installed in the Airbus A350-900. The Airbus supplier, Saft, designed a system with four batteries, each composed of 14 cells delivering 25V nominally combined. Thus, the A350-900 uses more batteries, with less power demanded from each cell than the 787 system.

The B787 problems were sufficiently fixed by providing more insulation between the cells, and installing an overboard ventilation system as a backup. No more incidents since.

Early A350s actually used NiCad batteries instead for exactly this reason, though they did switch to Li-ion for new deliveries about a year after the first frames were delivered.

Why are Li-ion batteries not being used in aircraft

Actually they are.  They are widely used in model airplanes, "drones", etc, which the (U.S.) Federal Aviation Administration refers to as "Unmanned Aircraft Systems".  See for example this link.

Which is, incidentally, why flight times on these are very short compared to petroleum-fueled aircraft. It makes a lot more sense for light UAVs, though, as electric motors can be easily scaled down to the very small sizes needed for these UAVs that weigh not more than a few pounds, whereas internal combustion engines can't scaled down that low quite as easily (though larger model airplanes do indeed commonly use ICE engines.) The flight times on most Li-ion powered UAVs literally wouldn't be enough to even meet the legally required reserves on manned flights. It would be illegal to take off.

Lithium Ion main ship batteries (non-propulsion) have been making their way into general aviation, helicopters, and business jets over the last 5 - 10 years.
Certification of new technology in aircraft almost always takes a significant amount of effort and time. After the issues Boeing experienced on the 787, the industry went through a period of scrutiny and enhanced requirement development.

Lithium Polymer (LiPo) batteries, are successfully used in smaller aircraft.
Larger aircraft require a non-linear increase in power.
I would expect batteries to be especially suitable for lighter than air applications such as airships.
Apart from energy density, batteries do not shed mass as they discharge in the same way a fuel tank might.
This does not mean we cannot use them in a hybrid fashion, with a low capacity installation ultimately powered from turbofans with generators for takeoff or emergency power.

"Larger aircraft require a non-linear increase in power." Do they? AFAIK, L/D ratio is actually significantly better on very large aircraft like airliners vs. most light singles, for example.

You need more than double the surface area to double the payload.  Air is proportionally less dense as we scale up. Smaller aircraft can afford to be less aggressively designed. The airliner advantage is the 8m sweet spot (or thereabouts) for turbofans.


text:
Should I hold my breath in case of a decompression?
If I'm, for example, in the lavatory where there are no oxygen masks, would it be advisable to hold your breath until you can reach an oxygen mask, or until the pilot brings the aircraft down to an altitude where we can breath again?
Is it even physically possible to hold your breath after a  decompression accident?

I think there might actually be masks in the lavatories.

It would probably be impossible for you to hold your breath, the pressure difference would be too great and, it you tried, you would cause yourself serious injury.

@Notts90 the FAA decided they had to be removed.

@juanCarlosSantana US airlines put the masks back in recently, last year someone I believe, but they were gone for a while.

Seems daft not to have them to me but not me that makes the rules!

@juanCarlosSantana I was on a flight in the US a week ago and I specifically remember the flight attendant mentioning oxygen masks in the lavatory compartment during the safety briefing.

2011, the FAA ordered the airlines to disable them. 2012, the FAA ordered the airlines to put them back again within 37 months. The new ones are tamper-proof, so you can't disable it easily to use the oxygen to start a fire.

Even if you could hold your breath despite the high internal pressure trying to force itself out, it would only help if you managed to take your breath in prior to the depressurization. Otherwise, you're just holding in an oxygen-sparse gas. As a result, the oxygen concentration difference between your blood and your lungs will equalize, but not in your favor. It'll pull oxygen OUT of your body. That's what's so dangerous about depressurization. It's not just about the lack of oxygen, but the secondary effect that has on sucking out the oxygen already in you.

No. Holding your breath might cause breaking blood vessels in your lung (barotrauma). It is important to equalize the pressure inside your lung with the exterior. That is in fact the most fundamental rule for all scuba divers.
Also see this related question: Is it possible to suffer barotrauma during decompression?

The answer by bogl is in the right line, also, I don't know if a person with their lungs having air at a higher pressure than the surroundings, as in a fast decompression, would be able to: 'hold breath' at all. The images in: 'Outland', a 1981 Sci-Fi movie featuring Sean Connery, are eloquent, but fantastic, unrealistic, this couldn't happen.


text:
Why do aircraft fly over Paris when they're forbidden from doing so?
I though aircraft were forbidden to fly over Paris, but I can hear a lot of planes flying over Paris, and, looking at Flightradar24, they are really flying over the city.
They are commercial planes, not military or private ones.
Do you know if the regulations surrounding this have changed, or if it's just something normal?

This is a map from SkyVector with the airspace around Paris:

You can see several major airports like Charles De Gaulle and Le Bourget to the North East and Orly to the South. In the center, there is a region marked LFP-23. This is the restricted airspace you probably had in mind. The French AIP defines it as follows:

LF P 23 PARIS
SFC to 6500ft AMSL
Excluding LF-P 47 BALARD, which retains its status.
CAG/CAM: prohibited penetration, including unmanned aircraft, with the exception of:

after agreement of the Préfecture de Police de Paris, aircraft on mission of emergency medical service, civil security, gendarmerie, state transport customs and defense for which bypassing is not compatible with the execution of the mission ;
defence aircraft on air security missions (notification to the Préfecture de Police de Paris as soon as possible) ;
aircraft that have applied for entry to the police headquarters in Paris at least five working days before the date of the flight and have obtained formal authorization ;
aircraft authorised by the Paris-Issy-les-Moulineaux Heliport Control Authority (LFPI) which follow departure and arrival procedures published by means of aeronautical information ;
twin-engine helicopters, in the event of engine failure at the take-off of the Paris-Issy-les-Moulineaux heliport (LFPI) in accordance with the regulations in force ;
flights authorised in accordance with the procedures described in AIP ENR 1.2 and aeronautical chart 1/100 000 "Itineraires Hélicoptères" in CTR PARIS (appendix of 8 February 1984 decree).

You therefore shouldn't see any commercial aircraft flying in this restricted airspace below 6500 ft AMSL. Above that and outside of the marked region, the restriction does not apply.

(+1) Note that the marked region roughly corresponds to what the locals consider to be Paris proper, the municipality of Paris (without the Bois de Vincennes and the Bois de Boulogne).

Also, just as a note for those not particularly familiar with commercial aviation: For commercial aircraft departing CDG, getting above 6,500' before getting to central Paris will definitely not be a problem. For example, at this particular moment, there is an AF flight from CDG to GNV that was passing 11,000' as it entered LFP-23. A DHL aircraft passing through the area from Casablanca on the way into CDG was still above 12,000'. The 6,500' rule more just keeps low-flying sightseeing flights and general aviation away from the city.

what's lfp 47? it's marked with same green color in same green circle-ish blob as lfp 23

It also looks like the entire CZ is in LFR-205

@NooneAtAll LFP-47 is listed as "BALARD: SFC to 2000 ft AMSL, Defence facilities, Entry prohibited for all ACFT, including unmanned aircraft except for twin-engines HEL, in case of failure of engine for take off from HLP Paris-Issy-les-Moulineaux according to the regulatory requirements in force." So there are fewer exception for flying in this airspace compared to the surrounding LFP-23.

@NooneAtAll: P47 includes the Defense ministry and Armed forces HQ. The green area behind in the article picture is Issy public heliport mentioned by Bianfable, and, visible on this picture, between them the beltway around Paris "intra-muros" (within fortifications). Paris, which delineates P23. Entering P23 (ambulances are allowed) requires a twin-turbine helicopter, but P47 is prohibited (including rotorcraft) except emergencies for rotorcraft taking off from Issy and experimenting a turbine failure.

@StephenS: R 205/2, /3, etc delineate Paris TMA at different altitudes.